The present invention relates to carrier particles for use in a two-component dry-type developer for developing latent electrostatic images to visible images, for use in electrophotography, electrostatic recording methods and electrostatic printing methods, and more particularly to carrier particles coated with a silicone layer containing, as an inorganic fortifying agent against abrasion, silicone carbide or potassium titanate.
Conventionally, as a developer for use in these fields, the so-called two-component dry-type developer is known, which comprises carrier particles and toner particles. In the two-component dry-type developer, the particle size of the toner particles is very much smaller than the particle size of the carrier particles, and the toner particles are triboelectrically attracted to the carrier particles and are held on the surface of the carrier particles. That electric attraction between the toner particles and the carrier particles is caused by the friction between the toner particles and the carrier particles. When the toner particles held on the carrier particles are brought near or into contact with a latent electrostatic image, the electric field of the latent electrostatic image works on the toner particles to separate the toner particles from the carrier particles, overcoming the bonding of the toner particles to the carrier particles, with the result that the toner particles are attracted towards the latent electrostatic image and the latent electrostatic image is thus developed to a visible image. In the case of the two-component dry-type developer, only the toner particles are consumed as the development is performed. Therefore, it is necessary to replenish the toner particles in the course of repeated development by the apparatus.
Further, it is necessary to triboelectrically charge the toner particles to the desired polarity and with a sufficient amount of charges, and to maintain the amount of the charges and the polarity for a long period of time during use. In the case of a conventional two-component type developer, it is apt to occur that the resin contained in and released from the toner particles in the course of the mechanical mixing of the toner particles and the carrier particles in the development apparatus eventually covers the surface of the carrier particles. Once the surface of the carrier particles is covered with the resin, which is generally referred to as the "spent phenomenon," such carrier particles no longer function as active carrier particles, that is, carrier particles which contribute to development. As a result, the charging characteristics of the carrier particles in the conventional two-component type developer deteriorate with time while in use. In the end, it is necessary to replace the entire developer by new developer.
In order to prevent the spent phenomenon, a method of coating the surface of carrier particles with a variety of resins has been proposed. However, resins which can in fact prevent the spent phenomenon have not been discovered. At one extreme, for instance, carrier particles coated with a styrene methacrylate copolymer or polystyrene are excellent in their triboelectric charging properties. However, since the surface energy of the carrier particles is comparatively high, the carrier particles are easily covered with the resin contained in the toner particles while in use. As a result, the spent phenomenon occurs easily and, accordingly, the life of such developer is short.
As carrier particles coated with a coating layer with a low surface energy, for example, the following carrier particles which are coated with a layer containing a silicone resin having a low surface energy have been proposed:
Carrier particles coated with a coating layer in which an unsaturated silicone resin, an organosilicon, silanol and other resins are mixed with a styrene-acrylic resin, as disclosed in U.S. Pat. No. 3,562,533; PA0 Carrier particles coated with a coating layer comprising a polyphenylene resin and an organosilicon-containing terpolymer resin, as disclosed in U.S. Pat. No. 3,847,127; PA0 Carrier particles coated with a coating layer comprising a styrene-acrylate-methacrylate resin, organosilane, silanol, siloxane and other resins, as disclosed in U.S. Pat. No. 3,627,522; PA0 Carrier particles coated with a coating layer comprising a silicone resin and a nitrogen-containing resin having positive charging characteristics, as disclosed in Japanese Laid-Open Patent Application No. 55-127567; and PA0 Carrier particles coated with a coating layer comprising an acrylic-resin-modified silicone resin, as disclosed in Japanese Laid-Open Patent Application No. 55-157751.
However, the silicone resins employed above are relatively weak in terms of mechanical strength. Therefore, if the carrier particles coated with such silicone resins are used with vigorous mixing, for example, in a development apparatus of a high speed copy machine, the coated layer is scratched or scraped off by collisions with the other carrier particles, the inner wall of the development apparatus and the surface of a photoconductor installed in the development apparatus. Eventually, the coated layer is removed from the carrier particles. As a result, the triboelectric charging of the developer occurs, not between toner particles and the silicone resin layer of the carrier particles, but between toner particles and the core material of the carrier particles. This change in the charging mode changes the quantity of electric charges of the developer and the image quality deteriorates.
In order to eliminate that shortcoming of the above silicone resins, methods of adding a variety of additives to fortify the silicone resins against abrasion have been proposed.
However, no additives which are capable of satisfactorily fortifying the silicone resins against abrasion, without changing the triboelectric charging performance of the carrier particles in respect of the toner particles, have been proposed.